With the recent trend toward weight reduction and size reduction in electrical products, the desire for development of a lithium secondary battery having a high energy density is becoming stronger than before. There also is a desire for improvements in various cell characteristics as a result of the spread of fields to which lithium secondary batteries are applied.
Nonaqueous-electrolyte-solution secondary batteries presently proposed employ a metal oxide salt such as LiCoO2, Li2Mn2 O4, or LiNiO2 for the positive electrode and further employ lithium metal or a compound capable of occluding and releasing lithium ions, such as a carbonaceous material, e.g., coke, artificial graphite, or natural graphite, or a metal oxide material, e.g., an oxide of Sn or Si, for the negative electrode.
In those nonaqueous-electrolyte-solution secondary batteries, ethylene carbonate is frequently used as the main solvent for the electrolyte solutions because of the high permittivity thereof. However, since ethylene carbonate has a high solidifying point, is solid at room temperature when used alone, and has a high viscosity, the electrolyte solutions employing ethylene carbonate as a solvent usually are ones in which the ethylene carbonate is used as a mixed solvent containing as a co-solvent a low-viscosity solvent such as a dialkyl carbonate, e.g., diethyl carbonate. However, since low-viscosity solvents generally have a low boiling point and a low permittivity, addition thereof in a large amount not only reduces the degree of dissociation of the lithium salt, resulting in reduced electrolyte solution performances, but also poses problems concerning salt precipitation caused by solvent volatilization, safety due to a lowered flash point, etc. Conversely, addition in too small an amount poses problems concerning low-temperature electrical conductivity and viscosity.
On the other hand, lactone compounds such as γ-butyrolactone have a sufficiently high permittivity, although inferior to ethylene carbonate, and have a low solidifying point and low viscosity. Such lactone compounds can hence exhibit sufficient electrolyte solution performances without being mixed with a low-viscosity solvent. As a result, such lactone compounds are excellent solvents which compare favorably in performance with electrolyte solutions employing a solvent obtained by mixing ethylene carbonate with a low-viscosity solvent.
Consequently, an electrolyte solution employing γ-butyrolactone as the main solvent and containing as a co-solvent about from 15 to 35% by volume ethylene carbonate and a nonaqueous-electrolyte-solution secondary battery employing this electrolyte solution have been proposed (Japanese Patent Laid-Open No. 31525/1999).
However, the electrolyte solution employing γ-butyrolactone is inferior in electrochemical oxidation resistance and reduction resistance to the electrolyte solutions employing a solvent obtained by mixing ethylene carbonate with a low-viscosity solvent. The γ-butyrolactone-based electrolyte solution hence has problems concerning, e.g., cell capacity retention at high temperatures, and a further improvement has been desired.